Generally, vehicle companies develop different types of power trains for automatic transmissions for vehicles. A 4-speed or 5-speed automatic transmission is usually adopted, and a 6-speed automatic transmission has also been developed and applied to vehicles. Recently, efforts have been made to develop a 7-speed automatic transmission.
For example, by assembling a single-pinion planetary gear set and a Ravineaux-type compound planetary gear set through three clutches and three brakes, seven speeds can be realized. More concretely, a single-pinion planetary gear set is disposed in a rear portion of a compound planetary gear set. A first planetary carrier of the single-pinion planetary gear set is directly connected to an input shaft such that the first planetary carrier always acts as an input element. A first clutch connects a first ring gear and a third sun gear of the compound planetary gear set, and a second clutch connects the first ring gear and a second sun gear of the compound planetary gear set.
A third clutch, operating as a variable input element, connects the second and third planetary carriers of the compound planetary gear set to the input shaft. At the same time, the second and third planetary carriers of the compound planetary gear set are variably connected to a transmission case through a first brake to selectively operate as a reaction element. A third ring gear of the compound planetary gear set is connected to an output gear to operate as an output element.
In addition, the second and third planetary carriers of the compound planetary gear set are connected to the transmission case by a one way clutch OWC such that a rotation of the second and third planetary carriers can be prevented, and a second brake is disposed between the transmission case and the second sun gear of the compound planetary gear set so that the second sun gear of the compound planetary gear set can operate as a selective reaction element. A first sun gear of the single-pinion planetary gear set is connected to the transmission case by a third brake so that the first sun gear of the single-pinion planetary gear set can operate as a selective reaction element.
In the power train as described above, a first speed is achieved by operating the first clutch and the first and third brakes. A second speed is achieved by canceling the first brake from its operation in the first speed, and operating the second brake. A third speed is achieved by canceling the second brake from its operation in the second speed, and operating the second clutch. A fourth speed is achieved by canceling the second clutch from its operation in the third speed, and operating the third clutch. A fifth speed is achieved through canceling the third brake from its operation in the fourth speed, and operating the second clutch. A sixth speed is achieved by canceling the operation of the first clutch, and operating the third brake. A seventh speed is achieved by canceling the second clutch from its operation in the sixth speed, and operating the second brake. Finally, reverse achieved by operating the second clutch, and the first and third brakes.
Although such a power train realizes seven forward speeds and a reverse speed, its adoption of a Ravineaux-type compound planetary gear set that is very large in the radial direction results in drag torque, and involves many operating elements in shifting. Additionally, the endurance of the planetary gear set deteriorates, and productivity and design freedom cannot be effectively obtained.